During the proposed and gradual conversion in the United States to the metric system, it is expected that two sets of rotary tools such as wrenches for driving threaded members such as nuts and bolts will be required, one set sized in the metric system and the other set sized in the English system. In certain cases, the use of a wrench or the like sized in one system to turn a bolt or nut having a head sized in the other system will cause damage to the bolt by rounding or scoring its driving head. This, for example, might occur if a 13/16 inch wrench were employed to turn a 19 mm. bolt head.
It would be desirable to provide a single rotary tool such as a socket wrench or box end wrench which would not only fit both a given English sized and a given metric sized nut or bolt, but which would, in addition, grasp the bolt or nut head with sufficient contact as to avoid scoring or turning down of the head.